Intracellular free calcium levels and pH will be measured in molluscan central neurons from Archidoris monteryensis. Calcium levels will be measured by using the metallochrome dyes, Arsenazo III and Antipyralazo III. Intracellular pH will be monitored both colorometrically and by using pH microelectrodes. Experiments will be run to quantify (Ca ions) and pH under resting conditions in reidentifiable neurons and to estimate the changes in these quantities which occur during electrical activity, action potentials and voltage clamp pulses, and during exposure to agents which modify cell activity, such as phosphodiesterase inhibitors, cAMP and ATP. The interrelation between pH and (Ca ions), will be explored by imposing changes in internal pH and determining the resulting changes in (Ca ions). The effects of internal pH on the Ca-activated potassium conductance will be studied. The dynamics of calcium reuptake following activity-induced loads will be examined before and after treatments which putatively block one or the other of the Ca sequestering or transporting mechanisms of the cell.